This present invention relates to thermoplastic compositions utilizing polymers of monovinylaromatic compounds which have been modified with rubber to increase their impact strength and which are particularly useful for manufacturing articles requiring increased environmental stress crack resistance (ESCR). More particularly, the present invention discloses a high impact polystyrene (HIPS) material which is particularly advantageous for use in food product containers which are normally subject to environmental stress cracking.
It is well known that rubber-reinforced polymers of monovinylaromatic compounds, such as styrene, alphamethyl styrene and ring-substituted styrenes are desirable for a variety of uses. More particularly, rubber reinforced polymers of styrene having included therein discrete particles of a crosslinked rubber, for example, polybutadiene, the discrete particles of rubber being dispersed throughout the styrene polymer matrix, can be used in a variety of applications including refrigerator linings, packaging applications, furniture, household appliances and toys. The conventional term for such rubber reinforced polymers is xe2x80x9cHigh Impact Polystyrenexe2x80x9d or xe2x80x9cHIPSxe2x80x9d. The physical characteristics and mechanical properties of HIPS are dependent upon many factors, including the particle size of the cross-linked rubber particles. One of the most desirable characteristics of HIPS materials is the ability of such materials to resist degradation or destruction by factors such as contact with foods and edible oils. In addition, other properties which must be maintained for such articles include flexural strength and tensile strength.
The property of stress crack resistance, or environmental stress crack resistance (ESCR), is particularly important in thermoplastic copolymers utilized in food containers. The food content of such polymer containers might not normally degrade the type of polymeric material of which the container is made, but when a thermoplastic polymer is thermoformed from extruded sheet material, residual stresses are locked into the molded article. These stresses open the polymer up to attack by substances which it would normally be totally resistant to. Such articles made from styrene polymers modified with rubber to increase impact strength are prone to stress cracking when they come into contact with common agents found in organic food products such as fats and oils. Likewise, such products are also subject to stress cracking when coming into contact with organic blowing agents such as halohydrocarbons containing fluorine and chlorine. These polymers generally are found in household items such as refrigerator liners, which may crack when the cavities in the refrigerators are filled with a polyurethane foam as a result of the blowing agent utilized in the foam.
In the past, environmental stress cracking has been prevented by complex procedures usually involving multiple layer polymer construction wherein an intermediate protective layer of polymer is placed between the polystyrene layer and the blowing agent or the fatty food materials. One such layer of material utilized to insulate the styrene from these agents is the terpolymer material known as ABS, or acrylonitrile-butadiene-styrene. Other attempts to improve the stress crack resistance of high impact monovinylaromatic polymers have been to increase the amount of rubber mixed in the polymer. Unfortunately, the higher rubber content decreases the tensile and flexural strengths of the final material. Other solutions have involved tightly controlling process conditions to maintain strict control over the particle size of the rubber particles cross-linked within the polystyrene matrix. One such patent disclosing this technique is that assigned to the assignee of the present invention, U.S. Pat. No. 4,777,210, issued Oct. 11, 1988, in which a continuous flow process for producing high impact polystyrene and for providing reliable and reproducible methods for varying particle sized was disclosed. In that patented process, a pre-inversion reactor was utilized to convert a solution of styrene, rubber (such as polybutadiene) and a peroxide catalyst into a high impact polystyrene material exhibiting high environmental stress crack resistance.
Another attempt to improve stress crack resistance was that disclosed in U.S. Pat. No. 4,144,204 to Mittnacht, et al., dated Mar. 13, 1979 in which a monovinylaromatic compound was modified with rubber to increase the ESCR and wherein the amount of rubber dissolved in the monomer prior to polymerization was chosen so that the content of the soft component (gel phase) in the impact resistant polymer was at least 28% by weight and preferably 38% by weight or more, based on the weight of the impact resistant polymer. The upper limit of the content of soft component was about 50 to 60% by weight and a preferable range of 30 to 40% by weight was found advantageous.
A third method used conventionally to increase ESCR in HIPS is that disclosed in British patent specification 1,362,399 in which a liquid hydrocarbon telomer having an unsaturated carbon chain is added to the HIPS material in amounts ranging from 0.2 up to 5 parts per hundred. Telomers are defined in Websters unabridged dictionary as the products of chemical reaction involving the addition of fragments of one molecule (such as alcohol, acetal or chloroform) to the ends of a polymerizing olefin chain. In the British patent, the telomers utilized exhibited number average molecular weights in the range of 1000 to 6000. Experiments attempting to utilize low molecular weight polybutadienes to manufacture ESCR-HIPS have been unsuccessful because of cross-linking, indicating that this patented process utilizes butadienes which are compounded or blended with polystyrene rather than being added during the polymerization reaction.
Another attempt to improve stress crack resistance of HIPS material can be found in British patent No. GB 2,153,370A, wherein a HIPS material was manufactured utilizing a high molecular weight rubber material having a stated molecular mass of at least 300,000, a viscosity greater than or equal to 140 centipoise; the resulting HIPS containing between 7 and 10% by weight of rubber, and the polymerization being carried out in the presence of alphamethyl styrene dimer or a compound chosen from n-dodecylmercaptan, tertiarydodecylmercaptan, diphenyl 1,3 butadiene, or various other compounds of mixtures thereof. Also, this process was carried out in the presence of cyclohexane and ethylbenzene equal to at least 7% by weight of the total ingredients. In addition, additives including monotriglycerides of stearates and polyethylene waxes were also necessary.
On the other hand, additives are used for reasons besides ESCR improvements. U.S. Pat. No. 3,506,740 to Dempsey, et al. teaches the use of low molecular weight polyolefins as internal lubricants for impact polystyrene compositions. Listed examples include polypropylenes and polybutylenes with molecular weights in the range of 800 to 1600 (as measured by vapor pressure osmometry).
It has also been discovered that the final properties of polymeric materials such as their molecular weight, can be affected by the amounts and types of polymerization initiators used. For example, U.S. Pat. No. 5,266,603 discloses the manufacture of expandable polystyrene in bead form having low benzene residual content using perketal and/or monoperoxycarbonate initiators.
U.S. Pat. No. 4,861,827 to Sosa, et al. discloses a styrene polymerization process for making high impact polystyrene utilizing a free radical initiator which decomposes during the polymerization process to form only non-acid decomposition by-products which can be left in the recycle stream and will not inhibit further polymerization. Examples of such free radical initiators include azo and peroxy compounds.
U.S. Pat. No. 5,559,162 teaches methods of making polymeric peroxycarbonates and processes for using the same as initiators. U.S. Pat. No. 4,433,099 teaches the use of organic peroxide initiators to manufacture impact-resistant styrene polymers with Mv molecular weights in the range of 150,000-170,000. U.S. Pat. No. 4,129,703 discloses the use of a peroxycarbonate initiator for making polystyrene in two thermally distinct polymerization stages, each between about 70xc2x0 C. and 150xc2x0 C., with a different initiator for each stage.
The present invention overcomes the disadvantages of the known high impact polystyrene materials by providing a polystyrene material (PS) containing a polybutadiene or styrene-butadiene rubber, which PS material has been polymerized with said rubber using a particular combination of polymerization initiator materials that provide improved ESCR properties over conventional PS initiators. The high impact polystyrene (HIPS) disclosed by the present invention demonstrates a high resistance to environmental stress cracking, high impact strength, good tensile strength and good flexural strength.